Teletext is the generic term for the transmission of alpha-numerical information via picture lines of a television signal, which lines are not used for picture information. The quantity of information which can be displayed on the display screen of a television receiver is generally referred to as teletext page. A transmitted page comprises a coded page number and a series of displayable characters which are transmitted in the form of digital character codes. A television station transmits a plurality of teletext pages having a large variety of information such as news, weather forecasts. TV programs, sports results, exchange rates, regional information, entertainment information, etc. The user can select each page by means of its page number. To facilitate access to the searched information, the information is generally clustered in groups and some teletext pages comprise the table of contents of such a group. These pages, also called index pages, refer to the page numbers on which the searched information can be found.
U.S. Pat. No. 4,992,871 describes a television receiver which is provided with positioning means for indicating a teletext page number displayed on the display screen. The relevant page number is read and applied to the teletext decoder in order that this decoder acquires the corresponding teletext page. In this way the user is offered an attractive alternative to the operation of manually entering the individual digits of the page number.
When the user selects a page number, the teletext decoder searches the pages corresponding to this number in an autonomic manner. Such a teletext decoder is described, inter alia in the book "Teletext and Viewdata" by Steve A. Money, Butterworth & Co, 1979. As has been described in this book, the decoder comprises an acquisition circuit which continuously compares the selected page number with the transmitted page numbers. To this end the page numbers are coded in a specific way and accommodated in a prescribed time lock of predetermined picture lines of the television signal. If the transmitted page number corresponds to the selected page number, a memory is addressed in response to which the full page is stored in this memory for immediate or later display.
Generally, a user selects teletext pages intentionally. He calls the pages which are interesting to him and of which he knows in advance that they are available, either because they are always transmitted, or because he has been informed about their presence by means of an index page. Nevertheless, the user often remains ignorant of the presence of teletext pages with information which may be interesting to him. He is actually dependent on the tact whether the relevant page numbers have been stated. He will never see such pages, unless arbitrary browsing accidentally informs him of their presence.